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Versions: 00 01 02 03 04                                                
NETWORK Working Group                                      Bernard Aboba
INTERNET-DRAFT                                                 Microsoft
Category: Informational
29 September 2001

                          The Mini-DHCP Server

This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC 2026.

Internet-Drafts are working documents of the Internet Engineering Task
Force (IETF), its areas, and its working groups.  Note that other groups
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inappropriate to use Internet-Drafts as reference material or to cite
them other than as "work in progress."

The list of current Internet-Drafts can be accessed at

To view the list Internet-Draft Shadow Directories, see

Copyright Notice

Copyright (C) The Internet Society (2001).  All Rights Reserved.


Today, with the rapid rise of home networking, there is a need for
simple mechanisms of IPv4 address allocation and name resolution. This
document describes the behavior of the mini-DHCP server, a small scale
DHCP server that is typically implemented as part of a home gateway.

As described in this document, the mini-DHCP server is capable of
allocating addresses either in single or multi-segment networks. It
supports dynamic DNS, and is capable of automatically detecting the
presence of a full-fledged DHCP server, or other mini-DHCP servers, and
shutting down as required.

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Table of Contents

1.     Introduction ..........................................    3
   1.1       Terminology .....................................    3
   1.2       Requirements language ...........................    3
2.     Overview ..............................................    4
   2.1       Resilience ......................................    4
   2.2       Dynamic DNS support .............................    4
   2.3       Compatibility with existing DHCP servers ........    4
   2.4       Bridged networks ................................    5
3.     Addressing ............................................    6
   3.1       Address allocation ..............................    6
   3.2       Address selection ...............................    7
   3.3       Multi-segment address allocation ................    7
4.     References ............................................    9
5.     Security considerations ...............................   10
6.     IANA considerations ...................................   10
Acknowledgments ..............................................   11
Author's addresses ...........................................   11
Intellectual Property Statement ..............................   11
Full Copyright Statement .....................................   11

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1.  Introduction

Today, home gateways frequently include functionality beyond that of a
router, as defined in RFC 1812 [10].  For example, home gateways
frequently support Network Address Translation (NAT), described in
[8]-[10], as well as acting as a DHCP server as described in RFC 2131
[3], and a DNS server as described in [13]. These small scale DHCP and
DNS servers will be described as "mini-DHCP" and "mini-DNS" servers
within this document.

While initial offerings were relatively simple devices, today's home
gateways are increasingly sophisticated.  For example, home gateways may
include support for multiple Internet or home interfaces, including
support for both 802.11 wireless [18], and wired networks.  This implies
that the mini-DHCP server may need to allocate addresses on multiple
segments, instead of just one.

In some cases, multiple home gateways may exist within the home, or a
home gateway may be brought into an enterprise environment, causing
potential conflicts between the mini-DHCP server and an existing DHCP

The purpose of this document is to provide guidance on how a mini-DHCP
server can behave so as to minimize the potential for conflict, and
maximize the services provided to users of the home network.

1.1.  Terminology

This document uses the following terms:

Site Administrator
          A Site Administrator is the person or organization responsible
          for handing out IP addresses to client machines.

DHCP client
          A DHCP client or "client" is an Internet host using DHCP to
          obtain configuration parameters such as a network address.

DHCP server
          A DHCP server or "server" is an Internet host that returns
          configuration parameters to DHCP clients.

1.2.  Requirements language

In this document, the key words "MAY", "MUST,  "MUST  NOT",  "OPTIONAL",
"RECOMMENDED",  "SHOULD",  and  "SHOULD  NOT",  are to be interpreted as
described in [1].

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2.  Overview

The mini-DHCP server provides DHCP server functionality, as described in
RFC 2131 [3], allocating IP addresses as well as providing for host
configuration. Among the host configuration information typically
provided by the mini-DHCP server is the address of the residential
gateway as well as the address of the mini-DNS server.

2.1.  Resilience

Since the mini-DHCP server will typically reside on a residential
gateway along with a mini-DNS server, it will typically provide its own
address in the default gateway and DNS server options, described in RFC
2132 [4]. However, in order to provide additional resilience, the
residential gateway can provide the addresses of secondary servers as
well.  For example, the residential gateway can obtain the addresses of
additional gateways (learned by listening to routing protocol
announcements on the home network interfaces) or DNS servers (learned
via PPP IPCP extensions [19] or DHCP [3] on the Internet interface).  In
the event that the home gateway is brought down (such as by a virus
attack), this additional configuration information can enable hosts to
continue accessing the Internet until the problem is resolved.

2.2.  Dynamic DNS support

The mini-DHCP server SHOULD support the functionality described in
"Resolution of DNS Name Conflicts Among DHCP Clients" [5], enabling
dynamic registration of the PTR and (if configured to do so) A records
for the hosts to whom it allocates addresses. Mini-DNS servers typically
do not support dynamic DNS update, so that the mini-DHCP server will
typically register both A and PTR records. The host provides its fully
qualified domain names to the mini-DHCP server via the DHCP Client FQDN
option, described in [6].

This allows the mini-DNS server to resolve DNS queries relating to hosts
on the internal network. Queries relating to Internet hosts are handled
by proxying the DNS query to the DNS server configured on the external
interface. This functionality is important, since multicast DNS, defined
in [17], is disabled by default when a DHCP server is available on the
network, and provides DNS server configuration. Thus, multicast DNS
cannot be relied upon to provide for name resolution in situations where
a mini-DHCP server is present.

2.3.  Compatibility with existing DHCP servers

In order to avoid conflicts with full-fledged DHCP servers, or other
mini-DHCP servers, it is necessary for the mini-DHCP server to
automatically determine whether it should be operating on an interface.

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A mini-DHCP server MUST NOT be active on an interface if there is
already a DHCP server active on that interface. Thus if the home
gateway's BOOTP relay agent has already been configured on an interface,
the mini-DHCP server MUST NOT be active on that interface.

In order to detect the presence of a DHCP server on interfaces that have
not been configured as BOOTP relay agents, a mini-DHCP server MUST
operate in promiscuous mode and send out periodic DHCPDISCOVER requests.
If a response is received, the mini-DHCP server MUST NOT provide DHCP
service on that interface.  Similarly, if the mini-DHCP server hears a
DHCPOFFER, DHCPACK or DHCPNAK on an interface, then it MUST NOT provide
DHCP service on that interface.

In the case where there is more than one mini-DHCP server active on a
segment, it is possible that the mini-DHCP servers will send
DHCPDISCOVER queries simultaneously, and thus without an election
mechanism, all of them might be shut down on an interface. As a result,
it is desirable to provide a deterministic method for deciding which
mini-DHCP servers shut down. As described in [20], the mini-DHCP
election option can be utilized for this purpose.

Note that a mechanism is needed to allow the mini-DHCP server to be
brought up again once the other DHCP servers are removed.  Once the
mini-DHCP server has detected another DHCP server and has stopped
offering service on an interface, it SHOULD set a timer. Once this timer
expires, the mini-DHCP server MUST once again send out a DHCPDISCOVER
and listen for responses.  The recommended timer interval is 5 minutes.

Note that if one or more DHCP servers are found on other interfaces, it
may not be desirable to run a mini-DHCP server on those interfaces
lacking a DHCP server. Instead, it may make more sense to operate those
interfaces in bridging mode as discussed in the next section.

2.4.  Bridged networks

Today mini-DHCP servers are typically included as part of home gateways
supporting Network Address Translation. However, the mini-DHCP server
can also improve ease of use in situations where routable address space
is available.

For example, the home gateway may be connected to an Intranet via a VPN,
or it may be attached via a dialup or broadband connection to an ISP
that operates its own DHCP server providing routable address space to
customers with attached LANs.

In these situations, it is possible for hosts on the home network to
obtain routable addresses from the ISP or Intranet DHCP server.  Rather
than acting as a mini-DHCP server and doing Network Address Translation,

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the home gateway can act as a bridge.  In this role the home gateway
forwards DHCPDISCOVER broadcasts down the link, but does not act as a a
BOOTP relay agent.

In order to enable automated detection of bridged versus NATed
operation, on bootup, the home gateway obtains an IP address on its
external interface and then sends a DHCPDISCOVER on that interface.  If
the external interface is a LAN link, and the original address was
obtained via DHCP, then a different client-identifier option must be
used in the subsequent DHCPDISCOVER. If the external interface is a PPP
link, then the home gateway an use the hardware address of its LAN
interface in the htype and chaddr fields.

On receiving one or more DHCPOFFERS, the home gateway configures itself
in bridging mode, and does not start the mini-DHCP or BOOTP relay
service on the external interface. It should be noted that in order to
properly route packets back to the attached home LANs, the upstream
router needs to keep track of the IP addresses assigned to the customer
hosts, and plumb corresponding static host routes on its interfaces.

Since ISPs operating in bridging mode typically do not provide unlimited
addresses, it is possible that the upstream DHCP server may stop
responding after a certain number of addresses have been allocated. In
this case it may be desirable for the home gateway to be able to act as
a bridge for those hosts that have obtained routable addresses, and a
router and mini-DHCP server for those hosts that are not able to do so.
However, doing this is tricky because it implies that two address
prefixes will co-exist on the home network segment. The home gateway
will need to act as a brouter, bridging traffic from the routable
addresses, while NAT'ing traffic from the private addresses allocated by
the mini-DHCP server. The home gateway will also need to forward traffic
from one prefix to the other on the home segment.

3.  Addressing

3.1.  Address allocation

By default, the mini-DHCP server configures itself to serve addresses
out of the 192.168/16 scope with /24 prefixes allocated to each

There are ISPs that use private address space internally in order to
manage network devices. Thus it is conceivable that a home gateway will
receive routing protocol announcements for a subnet of 192.168/16 on one
of its interfaces. Were the home gateway to listen to these
announcements, it is conceivable that it could become confused about the
routing topology.

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Thus home gateways implementing this specification MUST filter out
routing announcements for the 192.168/16 prefix on the Internet-facing

3.2.  Address selection

Since DHCP servers typically use static addresses, it is desirable for
the mini-DHCP server to have its IP addresses be per persistent between
reboots.  In order to choose an IP address on each interface, the mini-
DHCP server will operate as follows:

[a]  The mini-DHCP server will initially claim the .1 address on each
     interface (e.g.,, etc.), and then will
     attempt to determine whether the address is already allocated. This
     is accomplished by ARPing for the claimed address. If there is no
     response to the ARP, the mini-DHCP server will utilize the claimed

[b]  If the initially claimed address is taken, then the mini-DHCP
     server will derive the host portion of the address on each
     interface from the interface MAC address, and will claim and defend
     that address.  The formula for the computation of the host portion
     of the IPv4 address is as follows:

     host address = (0x'FFFF'  XOR netmask) && (CRC32 (MACAddr |
     hostname | interface-name ))

[c]  If both the initially chosen address and the computed address are
     taken, then the mini-DHCP server will choose a random address.

3.3.  Multi-segment address allocation

It is possible for home networks to include multiple segments. This
issue can  arise, for example, in the case of a home network supporting
802.11 wireless as well as IEEE 1394 and Ethernet.

In multi-segment small networks connected by a single router, it may be
desirable to provide for consistent IPv4 addressing in the case where
the small network has not been assigned a routable IPv4 address prefix.
The router may either be disconnected from the Internet, in which case
the hosts on the multiple segments will only be able to reach other, or
the router may offer Internet connectivity via Network Address
Translation (NAT), described in [8]-[10].

In order to enable effective IPv4 address allocation in multi-segment
networks connected by a single router it MUST be possible to
consistently assign addresses within multiple segments. Consistent

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addressing implies avoidance of address conflicts either within segments
or between segments. This consistency MUST be maintained in the event of
addition or removal of segments, or in the event of interfaces going up
or down.

In order to ensure consistency of addressing within multiple segments
connected to a single router, the mini-DHCP server MUST automatically
allocate /24 scopes out of the 192.168/16 prefix reserved for private
addressing, as described in [11], with a unique /24 prefix allocated to
each interface. Prefixes SHOULD be allocated from the bottom of the
range toward the top, starting with the 192.168.1/24 prefix.  The router
MUST NOT allocate the 192.168.0/24 or 192.168.255/24 prefixes, as these
are reserved for future use.

Note that in order to handle the case of interfaces coming up or down, a
scope MUST be allocated to each interface, whether it is functioning or
not. This allows a non-functioning interface to subsequently become
functional and to support consistent addressing. In the case where an
interface is added, such as by plugging in an additional card, a new
scope SHOULD be allocated as soon as the interface is added.

In order to allow for consistent numbering between router and host
reboots, scope assignments and address allocations should be handled as
required by RFC 2131 [3] with respect to use of stable storage.  Scopes
MUST NOT be de-allocated on interface-down or interface removal, so as
to remain robust against short term configuration changes.

To enable reclaiming of scopes in the event of permanent removal of an
interface, scope allocations of non-existent interfaces should timeout
using with an interval of three times the DHCP lease time. For example,
if the DHCP lease time is set to 3 days, then a scope allocated to a
removed interface will timeout (using an interval of three times) after
9 days.

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4.  References

[1]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
     Levels", BCP 14, RFC 2119, March 1997.

[2]  Droms, R., Arbaugh, W., "Authentication for DHCP Messages", RFC
     3118, June 2001.  July 2000.

[3]  Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, March

[4]  Alexander, S., Droms, R., "DHCP Options and BOOTP Vendor
     Extensions", RFC 2132, March 1997.

[5]  Stapp, M., "Resolution of DNS Name Conflicts Among DHCP Clients",
     Internet draft (work in progress), draft-ietf-dhc-ddns-
     resolution-02.txt, July 2001.

[6]  Stapp, M., Rekhter, Y., "The DHCP Client FQDN Option", Internet
     draft (work in progress), draft-ietf-dhc-fqdn-option-02.txt, July

[7]  Thomson,  S.,  Narten,  T.,  "IPv6  Stateless  Address
     Autoconfiguration", RFC 2462, December 1998.

[8]  Srisuresh, P., Holdrege, M., "IP Network Address Translator (NAT)
     Terminology and Considerations", RFC 2663, August 1999.

[9]  Srisuresh, P., Egevang, K., "Traditional IP Network Address
     Translator (Traditional NAT)", RFC 3022, January 2001.

[10] Holdrege, M., Srisuresh, P., "Protocol Complications with the IP
     Network Address Translator", RFC 3027, January 2001.

[11] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G. J., Lear,
     E., "Address Allocation for Private Internets", RFC 1918, February,

[12] Baker, F., "Requirements for IP Version 4 Routers", RFC 1812, June

[13] Mockapetris, P., "Domain Names - Implementation and Specification",
     RFC 1035, November 1987.

     1034, November, 1987.

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[15] Vixie, P., Thomson, S., Rekhter, Y., Bound, J., "Dynamic Updates in
     the Domain Name System (DNS UPDATE)", RFC 2136, April 1997.

[16] Eastlake, D., "Domain Name System Security Extensions", RFC 2535,
     March 1999.

[17] Esibov, L., Aboba., B., Thaler, D., "Multicast DNS", Internet draft
     (work in progress), draft-ietf-dnsext-mdns-07.txt, October 2001.

[18] Information technology - Telecommunications and information
     exchange between systems - Local and metropolitan area networks -
     Specific Requirements Part 11:  Wireless LAN Medium Access Control
     (MAC) and Physical Layer (PHY) Specifications, IEEE Std.
     802.11-1997, 1997.

[19] Cobb, S., "PPP Internet Protocol Control Protocol Extensions for
     Name Server Addresses", RFC 1877, December 1995.

[20] Akinlar, C., Braun, D., Mukherjee, S., "Mini-DHCP Election Option
     For DHCP", Internet draft (work in progress), draft-akinlar-
     zeroconf-minidhcp-option-00.txt, March 2000.

5.  Security Considerations

As noted in [2], DHCP is vulnerable to a number of threats, including
message modification and attacks by rogue servers and unauthenticated
clients. While the procedure described in this document does not
preclude implementation of DHCP authentication, the extra configuration
required to set this up represents an implementation barrier in the home
network. As a result, it is likely that most home routers will not
support DHCP authentication, and that those networks will remain
vulnerable to the attacks described in [2].

These threats are most serious in wireless networks such as 802.11,
since attackers on a wired network will require physical access to the
home network, while wireless attackers may reside outside the home. In
order to provide for privacy equivalent to a wired network, the 802.11
specification provides for RC4-based encryption. This is known as the
"Wired Equivalency Privacy" (WEP) specification, described in [18].
Where WEP is implemented, an attacker will need to obtain the WEP key
prior to gaining access to the home network.

6.  IANA Considerations

This draft does not create any new number spaces for IANA

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This draft has been enriched by comments from Ryan Troll of @Home and
Peter Ford and Yaron Goland of Microsoft.

Authors' Addresses

Bernard Aboba
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052

Phone: +1 (425) 936-6605
EMail: bernarda@microsoft.com

Intellectual Property Statement

The IETF takes no position regarding the validity or scope of any
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Full Copyright Statement

Copyright (C) The Internet Society (2001).  All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it or
assist in its implementation may be prepared, copied, published and
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may not be modified in any way, such as by removing the copyright notice
or references to the Internet Society or other Internet organizations,

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except as needed for the purpose of developing Internet standards in
which case the procedures for copyrights defined in the Internet
Standards process must be followed, or as required to translate it into
languages other than English.  The limited permissions granted above are
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successors or assigns.  This document and the information contained
herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE

Expiration Date

This memo is filed as <draft-aboba-dhc-mini-04.txt>,  and  expires April
15, 2002.

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